1. Field of the Invention
The present invention relates generally to a technical domain of a sliding element engaging a relative rotational motion. More particularly, the invention relates to a sliding element which reduces a friction coefficient on the sliding face and prevents a fluid from leaking from the sliding faces
2. Description of the Related Art
Related art of the present invention is found in U.S. Pat. No. 4,407,513 issued to Takenaka; U.S. Pat. No. 4,415,168 issued to Takenaka; U.S. Pat. No. 4,423,879 issued to Takenaka; U.S. Pat. No. 5,312,117 issued to Takenaka. These patents disclose a seal ring of a mechanical seal shown in FIG. 13. FIG. 13 is the prior art 1 related to the present invention.
FIG. 13 is an oblique view of the seal ring 110 which is one of a pair of seal rings used for a mechanical seal. This seal ring 110 is disposed in a mechanical seal for a compressor with a highly pressurized fluid which is typically used under a situation of varying temperature. In the mechanical seal, the seal ring 110 out of the pair of seal rings serves as either a rotary seal ring or a stationary seal ring. Also in this mechanical seal, a relative sliding motion between a sliding face of a rotary seal ring and a sliding face of a stationary seal ring provides a fluid-tight seal against the fluid which is located on either side between a rotary shaft and a housing.
There are eleven thin line grooves disposed on the sliding face 110A in an equally spaced manner along a circumference of the face which extend form the inner circumference towards the outer circumference and are inclined to an opposite direction relative to a rotary direction. The thin line grooves 115 do not reach the outer circumferential edge of the sliding face 110A, but have an open end towards the inner circumferential edge. It is described that the thus arranged thin line grooves 115 improves a seal performance against the fluid. However, for the small number of thin line grooves 115 disposed on the sliding face 110A, which is shown in FIG. 13, a fluid does not stay on the sliding face 110A of the seal ring 110 when the seal ring 110 is subjected to a low rotational speed. As the result, an increased sliding friction will cause abrasion, which in turn will lead to a leakage of the fluid.
There is the seal ring 155 shown in FIG. 14 as another prior art relative to the present invention, which is designated as the prior art 2.
This seal ring 155 is disposed in a mechanical seal as a stationary seal ring or/and rotary seal ring. As far as a fluid leakage problem is concerned, this seal ring 155 performs better than the prior art 1.
In this prior art 2, there are concave sections 156 disposed at the crossing points of radial directions and circumferential directions.
The form of the concave section 156 is elliptic when viewed from the top and the width of the minor axis is in the range of from 30×10−6 m to 100×10−6 m and the width of the major axis is in the range of from 60×10−6 m to 500×10−6 m. The concave section 156 is so arranged that the maximum width is a little greater than twice of the minimum width.
The concave section 156 tends to reserve a fluid inside the section which flows in between the sliding face 155A of the seal ring 155 and a sliding face of a stationary seal ring. The fluid flowing in from the outer circumference side of the seal ring 155 is trapped and reserved in the concave section 156 before the fluid reaches the inner circumference edge. The fluid retained in the concave section 156 is pushed toward the opposite direction relative to a rotational direction of the concave 156 due to a viscosity of the fluid and a rotary motion of the concave section 156, and a portion of the fluid beyond a reserving capacity of the concave section 156 starts to slip out from an outer circumferential edge of the concave section 156, which moves between the relatively sliding faces and is eventually trapped by an adjacent concave section 156. Thus, the fluid is pushed backward relative to a rotary direction of the sliding face 155A.
The concave section 156 of the sliding face 155A has an elliptic form and the length and width of the section 156 are set small. The longitudinal axis of the concave section 156 is inclined to an opposite direction relative to a rotary direction. And the number of the concave section 156 deployed is relatively small. Therefore, the concave sections 156 are not good enough to be able to trap and reserve a fluid on the sliding face 155A. Also the longitudinal length of the concave section 156 is so small that it cannot creates a sufficient pumping effect for pushing back the fluid towards a fluid reservoir. Thus, no significant seal performance is exhibited with the prior art. There is also a room for improvement in terms of a reduction in friction coefficient and frictional heat generation. For a slow rotational speed of the seal ring 155, in particular, a reduced lubrication effect makes it difficult to decrease the sliding friction.
There is another seal ring for a mechanical seal which is not shown in a figure but is similar to the one shown in FIG. 14 as another prior art relative to the present invention, which is designated as a prior art 3. On a sliding face of the seal ring for a mechanical seal, there are concave sections whose longitudinal direction coincides with a radial direction of the seal ring, and the concave sections are located at the crossing points of radial directions and circumferential directions. In the arrangement, the concave sections are disposed like a houndstooth check spreading in both a radial direction and a circumferential direction. Major axis of every concave section is aligned with the radial direction of the sliding face.
Since the major axis of the concave section coincides with the radial direction of the sliding face, as a rotation of the seal ring increases the more fluid is captured within the concave section, and as the result a dynamic pressure within the concave section gradually increases. Therefore a lubrication layer of the fluid is created on the sliding face by the fluid escaping from the concave section due to its increased dynamic pressure. The fluid within the concave section, however, cannot be preserved for a long period of time, especially under a low rotational speed. As a result, the friction coefficient of the sliding face will gradually increase in a long run under a low rotational speed. Also the lubricant escapes the concave section along a radial direction of the sliding face, which makes it difficult to improve the seal performance. Furthermore, the concave section which serves as a dynamic pressure inducing groove constitutes a contact-type seal under a low rotational speed and does not exhibit an ability of generating a dynamic pressure.
Technical problems related to these prior arts remain in that they suffer from a difficulty of creating a lubrication layer by drawing in a fluid for reducing a friction coefficient, and also that they find a difficulty in retaining a fluid once drawn in on the sliding face as well as in pushing back the fluid for securing a seal performance of the fluid. Such a difficulty on creating a lubrication layer will cause a heat generation on the sliding face.
In particular, a difficulty in improving a seal performance as well as decreasing a friction coefficient arises when the sliding element rotates at a slow speed and a fluid pressure becomes low.
A primary object of the present invention is to decrease a frictional resistance on a sliding face of a sliding element by retaining a lubricant fluid on the sliding face while the element is in rotation. Another object is to provide a lubrication layer on the sliding face for improving a seal performance. Yet another object is not only to prevent a heat generation of the sliding face of the sliding element during rotation but also to prevent the sliding face from wearing. Yet another object is to improve a seal performance as well as to decrease a frictional resistance even when a sliding element rotates at a low speed.